The heart of a computer's long term memory is an assembly that is referred to as a magnetic hard disk drive. The hard disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of the rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head is generally an inductive write element that includes an electrically conductive coil that generates a magnetic flux in a write pole. The read head includes a magnetoresitive sensor. In current read head designs, either a giant magnetoresistive (GMR) sensor, or a tunneling magnetoresistive (TMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a free layer which responds to an external field and a pinning layer whose polarity is fixed. The resistance value of the sensor changes with the relative polarity between the free layer and the pinning layer.
On a conventional disk, the magnetic surface of the disk is continuous. Binary information is recorded on the disk by polarizing a unit (called a bit) of the disk to be one polarity which representing a value of “1”. On the other hand the bit can be in the opposite polarity which represents the value of “0”. The smaller the bit, the more information can be stored in a given area. Present magnetic recording may achieve a bit as small as around 18×80 nanometers. Each bit includes multiple magnetic grains, and the typical grain size is about 6 nanometers. Therefore, in a bit of size 18×80 nanometers, there are about 40 grains.
To increase the areal density of the magnetic disk, the bit size is reduced. If the grain sizes are kept the same for smaller bit sizes, then there would be a smaller number of grains in a bit resulting in a smaller signal-to-noise ratio (SNR). If the grain sizes are reduced proportionally to keep the number of grains in a bit constant for smaller bit sizes, then the SNR would be the same. However, the super-paramagnetic effect would occur when grain sizes are reduced to too small a size since the magnetic grains on the disk become so tiny that ambient temperature can reverse their magnetic orientations over the course of the time. The result in such a reversal is that the bit is erased and the data is lost.
One solution to the problems posed by the super-paramagnetic effect is to pattern the magnetic disk. In magnetic patterned media, the magnetic material is patterned into small magnetically isolated blocks or magnetic islands and in each magnetic island there is only a single magnetic domain. The single magnetic domain can be a single grain or consist of a few strongly coupled grains that switch magnetic states in concert as a single magnetic volume. Each island serves as the basic storage element “bit”. This is in contrast to conventional continuous media wherein a single “bit” may have multiple magnetic domains separated by grain boundaries. U.S. Pat. No. 5,820,769 is representative of various types of patterned media and their methods of fabrication. A description of magnetic recording systems with patterned media and their associated challenges is presented by R. L. White et al., “Patterned Media: A Viable Route to 50 Gbit/in2 and Up for Magnetic Recording”, IEEE Transactions on Magnetics, Vol. 33, No. 1, January 1997, 990-995.
Patterned magnetic recording media, including Bit-Patterned-Media (BPM) is therefore a promising solution to overcome the super-paramagnetic limit facing continuous perpendicular and longitudinal media. In BPM, each bit consists of only one independent island which has a single magnetic domain, in contrast to continuous media where each bit consists of multiple grains. Each island though may have a different switching field (SF), which is the minimum field need to flip the polarity of an island from one to the opposite.
Bit-patterned media (BPM) needs a tight switching field distribution of the magnetic islands in a proper range to enable correct recording. If an SF of an individual island is higher than a peak field of a write head writing the island, the island will never be written by the write head. However, this issue can be addressed by using a write head with a sufficient peak field to write all the islands, or manufacturing a media with low enough SF for each island. However, even for an SF below the peak field of the write head, the SF distribution has to be tight enough to avoid an island being either not written successfully or being overwritten when writing adjacent or near by bits.
FIG. 3 graphically illustrates the situations where the write field is over both the island intended to be written 303 as well as its neighboring islands 304, 305. There are errors in writing the intended to write island 303, and in over-writing the island which had been written in previous write clock 304. The area A 301 of FIG. 3 indicates the population of islands which are at the high end of the SF distribution of the magnetic islands. Their switching fields are above the highest write field Hh employed to write a bit. Typically, a bit is written at a level Hh, which is lower than the peak write head field Hp. The resultant errors in area A (301) will be referred to as type A errors. An example of when an island is overwritten when writing the next bits is area B 302 of FIG. 3 and corresponds to bit 304. The switching fields of these islands are so low, below Hl, that the fringing field of the write head when writing the current island 303 will flip the polarities of the previous islands 304. These overwritten islands are from islands at the lower-end of the SF distribution and overwriting these islands will be referred to as type B errors. Both A and B type errors contribute to the error rate. If a stronger field is used to minimize the no-write error (area A 301), the over-written error (area B 302) will increase. Consequently, the total sum of errors of type A and type B will be increased. On the other hand, if a weaker field is used to minimize the over-written error (area B), then the no-write error (type A) will increase and consequently the sum of the errors of type A and type B (A+B) will be increased. A proper recording process minimizes the total write-in errors, i.e., the sum of A and B. The conventional process to minimize A+B is to achieve some compromise between A and B. However, in this compromise, neither A nor B is minimized. Although the sum A+B is the minimized, it is still far greater than the sum of A and B if both A and B were minimized. Therefore, a method and system is needed to further minimize the total errors A+B to the level that both type A and type B errors are jointly minimized.